On Wed, Jun 5, 2024 at 3:58 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> From: Frederic Weisbecker <frederic@xxxxxxxxxx>
>
> When the grace period kthread checks the extended quiescent state
> counter of a CPU, full ordering is necessary to ensure that either:
>
> * If the GP kthread observes the remote target in an extended quiescent
> state, then that target must observe all accesses prior to the current
> grace period, including the current grace period sequence number, once
> it exits that extended quiescent state.
>
> or:
>
> * If the GP kthread observes the remote target NOT in an extended
> quiescent state, then the target further entering in an extended
> quiescent state must observe all accesses prior to the current
> grace period, including the current grace period sequence number, once
> it enters that extended quiescent state.
>
> This ordering is enforced through a full memory barrier placed right
> before taking the first EQS snapshot. However this is superfluous
> because the snapshot is taken while holding the target's rnp lock which
> provides the necessary ordering through its chain of
> smp_mb__after_unlock_lock().
>
> Remove the needless explicit barrier before the snapshot and put a
> comment about the implicit barrier newly relied upon here.
>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
> kernel/rcu/tree_exp.h | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 8a1d9c8bd9f74..bec24ea6777e8 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -357,7 +357,13 @@ static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
> !(rnp->qsmaskinitnext & mask)) {
> mask_ofl_test |= mask;
> } else {
> - snap = rcu_dynticks_snap(cpu);
> + /*
> + * Full ordering against accesses prior current GP and
> + * also against current GP sequence number is enforced
> + * by current rnp locking with chained
> + * smp_mb__after_unlock_lock().
Again, worth mentioning the chaining sites sync_exp_reset_tree() and
this function?
Thanks
Neeraj
> + */
> + snap = ct_dynticks_cpu_acquire(cpu);
> if (rcu_dynticks_in_eqs(snap))
> mask_ofl_test |= mask;
> else
> --
> 2.40.1
>
>
